Disposable articles and their construction materials including fabrics, films, and adhesives are described in a variety of United States patents. Adhesives for such articles have been developed. In initial work, Korpman, U.S. Pat. No. 4,028,292, teaches a heat resistant adhesive material comprising a reactive phenol formaldehyde resin and a suitable antioxidant of a metal dithiocarbamate. Collins et al., U.S. Pat. No. 4,136,699, teach a disposable article using a hot melt ABA block polymer containing pressure sensitive adhesive as a positioning and construction material. Similarly, Chen et al., U.S. Pat. No. 4,460,364, teach a hot melt PSA used in the disposables. Schmidt, Jr. et al., U.S. Pat. No. 4,526,577, teach SBS block copolymers system in disposables using multiline application technology. Puletti et al., U.S. Pat. No. 4,627,847, also teach the use of hot melts in disposables. Tsukahara, U.S. Pat. No. 4,745,026, teaches a delayed tack sheet using an aqueous dispersion of, e.g., a polymer, a solid plasticizer and preferably a tackifier. Quinn et al., U.S. Patent Publication No. US 2003/0139516 A1 teach certain hot melt adhesives utilizing a very broad range of materials. Quinn et al., U.S. Pat. No. 6,582,829 teach certain hot melt adhesives utilizing an ethylene α-olefin (EAO) polymer. The application discloses a very broad range of materials. Dubois et al, U.S. Pat. No. 6,107,430 teach a broad range of proposed formulations and teaches a 1000 melt index ethylene octene polymer (one type of EAO) combined with a tackifying resin, a wax and an antioxidant material. Werenicz et al., U.S. Pat. No. 6,120,887 teach hot melt adhesive compositions using, on the whole, exemplary materials generally containing less than 40% of a low MI (high molecular weight) polymer material. Jialanella et al., U.S. Pat. No. 6,300,398 relates to an ethylene α-olefin polymer mixture with a wax and a nucleating agent to improve elongation at break of the polymer material. While the disclosure mentions the use of these materials in adhesives, no specific formulatory strategies are shown in the reference. Polymer compositions in the form of mixtures of materials are shown in the patent from Column 20, line 50 through Column 24, line 27. Dubois et al., U.S. Pat. No. 6,319,979 teach low application temperature hot melt adhesives including an ethylene α-olefin polymer material. The patent has a very broad disclosure, but discloses exemplary materials beginning at Column 25, line 62 through Column 30, line 36. As a whole, the materials have less than 33% of the ethylene octene polymer (having a 1000 gm-10 min−1 melt index). Table III in Column 28 further shows additional examples using certain polymeric materials with a melt index (MI) of 500 or 1000 grams-10 min−1 and in amounts less than 33 wt-%. Ahmed et al., U.S. Pat. No. 6,534,572 show compositions comprising a thermoplastic component and a superabsorbent polymer material. Kroll et al., U.S. Pat. No. 6,579,915 teach certain radiation crosslinked or curable hot melt adhesives utilizing low application temperatures. The application discloses a very broad range of materials, but uses a vinyl modified block polymer, KX-222CS. The vinyl substituent on the block polymer is used for radiation cross-linking.
In general, hot melt adhesives are formulated to contain at least a polymer and a tackifying resin, but can include a diluent, a stabilizer and other components. (Raykovitz, “Adhesive Coatings for Non-Wovens,” Proceedings of the 2nd International PIRA Conference, November 1989, p. 1.) The main function of the polymer is to provide strength as well as aid in adhesion. The tackifier promotes adhesion and wetting and contributes to adhesive tack. Commonly used tackifiers are rosins, modified rosins, terpenes, and hydrocarbons. The diluent promotes wetting and reduces the viscosity of the formulation. Mineral oil and wax are commonly used diluents. The stabilizer maintains viscosity, color, and physical properties as well as preventing thermal degradation. Hindered phenols, phosphites, and thioesters are commonly used stabilizers.
Ethylene-vinyl acetate copolymers (EVAs) are also widely used as adhesive resins along with tackifiers and plasticizers. Traditional hot melt blends incorporate copolymers having vinyl acetate content of 18 to 40% into formulations comprising 30 to 40 wt % EVA, 30 to 40 wt % tackifying resin, and 20 to 30% petroleum wax. (Skeist, I. ed., Handbook of Adhesives, 3rd ed., ©1990 by Van Nostrand Reinhold, N.Y., 408-22, 409.) The EVA provides strength and toughness to the adhesive, the tackifier provides surface wetting and tack, and the wax is used to lower the melt viscosity, reduce cost, and control speed of application of the adhesive. Often, plasticizers are included as well, depending on the application. While the melt index of EVA polymers ranges widely, traditionally these copolymers do not have melt index above about 500. Higher vinyl acetate content in the copolymer increases solubility, flexibility, hot tack, adhesion, and improves low temperature performance. Lower vinyl acetate content, however, leads to improved seal strength, increased resistance to blocking, and increased paraffin solubility while decreasing the properties boosted by higher vinyl acetate content. Thus, in any given application, a tradeoff is made with regard to vinyl acetate content depending on which properties are the most desirable or necessary.
A common group tackifiers employed in hot melt adhesive formulations with EVA is the rosin ester group. Because they impart compatibility to other components of a formulation, these tackifiers find utility as broadening the scope of other possible additives in an EVA formulation. Three basic types of rosin are available, all being natural products derived from tree byproducts. Gum rosin is derived from living pine trees, while wood rosin is harvested from aged pine stumps. Tall oil rosin is a byproduct of papermaking processes and as such has the largest source of supply. Unmodified rosins are highly unsaturated and thus are subject to degradation by UV, heat, and oxygen. Thus, industrially useful rosins are hydrogenated, disproportionated, and/or dimerized and then esterified. Glycerin and pentaerythritol are the most common esterification agents. Esterified rosins generally exhibit good adhesion, low color, good heat stability, and good aging stability.
Recently manufacturers have strived to meet new demands for hot melt adhesive technology by applying a number of different strategies to the traditional EVA hot melt formulation technology. The traditional strength of EVA hot melt adhesive technology is its relative low cost and wide range of utility. Improvements are continually made in this area, with an eye toward lowering cost or improving product performance, e.g. low temperature utility, increasing speed of application, lowering application temperature, increasing tack, etc. Specialty adhesives formulated for specific applications are also seen in the art.
In furthering the general utility of EVA based hot melt adhesives, Schmidt Jr. et al, U.S. Pat. No. 4,460,728, disclose that a combination of EVA and atactic polypropylene with tackifier, plasticizer, petroleum wax, and a stabilizer provide good processing and adhesive properties combined with a lower cost due to the inclusion of polypropylene. The formulation is employed in disposable diaper products. Korpman, U.S. Pat. No. 4,813,947 discloses a hot melt composition for bonding a tape tab to e.g. a diaper, such that the tab is reversibly adhered to a pressure sensitive adhesive; repeated opening and closing of the adhesive to tab does not result in failure of the adhesion of the tab to the diaper. The formulation employs an ABA block copolymer, a second polymer that may be EVA, a tackifier, and a hot melt modifying resin. In a similar application, Chen et al., U.S. Pat. No. 5,061,262 disclose a hot melt adhesive with an elongation of 100% at break to accomplish the same goal of retaining a tape on a diaper for repeated opening and resealing. The formulation employs two EVA polymers, one having 18-40 wt % vinyl acetate and the second having 14-20 wt % vinyl acetate; a tackifier, and microcrystalline wax.
Riswick et al., U.S. Pat. No. 5,804,519 disclose a method of improving the strikethrough properties of, among others, EVA based hot melt adhesives by adding 0.10 to 10 parts by weight of a fluorochemical surfactant based on the weight of the adhesive formulation. This advantage is useful in disposable absorbent articles, i.e. diapers and the like. The increased hydrophilicity of the adhesive enhances the transfer of liquid from the skin contact layer to the absorbent fluff layer underneath, where the adhesive resides between the layers. Similarly, Gibes et al., U.S. Patent Publication No. US 2002/0161085 discloses a hot melt adhesive with a hydrophile-lipophile balance (HLB) of less than 15, for the purpose of enhancing liquid transfer by the adhesive into the absorptive layers of a disposable absorptive article. Kauffman et al., U.S. Pat. No. 5,532,306 disclose a hydrophilic hot melt adhesive that will release upon exposure to moisture. The formulations optionally include EVA copolymer in addition to ABA lineal or radial block copolymer, and a water soluble or dispersible plasticizer, and other optional ingredients.
Simmons et al., U.S. Pat. No. 5,942,569 discloses the use of a pelletizing agent that can be added to tacky, pressure sensitive hot melt adhesives comprising EVA, among others, to provide for storage of the formulated adhesive in pellet form. The compositions comprise at least a thermoplastic polymer (e.g. EVA), tackifier, plasticizer, and a pelletizing aid that is, for example, polyethylene wax, polyamide wax, or a stearamide wax. Wang et al., U.S. Pat. No. 6,143,818 disclose a hot melt adhesive formulation that is a modified ethylene-propylene copolymer rubber (EPR) or ethylene-propylene-diene monomer copolymer rubber (EPDM), wherein the modification is to include a semicrystalline polymer such as EVA to the mixture of the rubber, along with a tackifier and optionally plasticizer, wax, stabilizer, antioxidant, and/or filler. The EVA provides cohesive strength to the formulation so that the addition of covalent crosslinkers, typically required for EPR and EPDM adhesives, is obviated. One disclosed use of these formulations is as a sprayable adhesive for elastic attachment in nonwoven disposable articles.
Many advances in EVA hot melt art are directed to specialty applications. For example, Colon et al., U.S. Pat. Nos. 4,743,238, 4,895,567, and 5,066,711 disclose hot melt adhesives that provides a wetness indicating property. The formulation can contain EVA, in addition to water sensitive polymers such as polyvinylpyrrolidone or, alternatively, a surfactant. Also required is an acidic material, typically an acidic tackifier, and a color-based wetness indicating agent. Svenningsen, et al., U.S. Patent Publication No. US 2002/0115744 A1 disclose a method of including imparting antimicrobial properties to a hot melt adhesive formulation that includes a polymer, for example EVA, tackifier, plasticizer, wax, antioxidant, and 0.01-5 wt % of a bacteriostat based on the weight of the formulation. Such adhesives find utility in disposable nonwoven products such as diapers.
Still other advances in EVA hot melt technology are directed to adhesion to specific substrates. Godfrey, U.S. Pat. No. 4,299,745, discloses an EVA hot melt formulation with enhanced adhesion to polyethylene substrates, where the addition of a modified polyethylene with saponification number of 3 to 60 provides enhanced adhesion properties. Other required ingredients are EVA copolymer, tackifier, and microcrystalline wax. Yeo et al., U.S. Pat. No. 5,695,855 discloses a printable hot melt adhesive specifically for printing a durable pattern onto a polyolefin nonwoven. The formulation contains EVA of a particular range of viscosity and a pigment. Vaughan et al., U.S. Patent Publication No. US 2006/0135694 A1 discloses a means to improve adhesion of EVA based hot melt adhesives to substrates such as polyethylene films as provided in disposable nonwoven application. The improved adhesion is provided in a formulation having, at minimum, EVA with 10-30 wt % vinyl acetate, styrene-ethylene-butylene-styrene (SEBS) block copolymer having a styrene content of 10-25 wt %, and a tackifier.
Despite the advances made in EVA based hot melt adhesives for use in disposable articles, there remains a substantial need in this art to obtain relatively inexpensive hot melt adhesives with excellent initial adhesion of components to film when used in disposable diaper applications. To this end, there is a substantial need to provide a formulation with a minimum number of ingredients, low cost and ease of blending ingredients. Such formulations can minimize error in amounts added, order of addition, etc. This provides ease of use that minimizes waste and maximizes efficiency yet still provides very high adhesion.
A substantial need also exists to provide adhesives having a stable level of adhesion, such that aging of the adhesive does not lead to a change in adhesion level. A substantial need also exists to obtain a thermally stable hot melt adhesive formulation that withstands long residence times at application temperature without suffering from changing viscosity due to crosslinking, degradation, or other undesirable events. A substantial need also exists to obtain a non-tacky, pelletizable hot melt adhesive formulation having the desirable properties listed above, so that the formulation is easily stored. A substantial need also exists for hot melt adhesive formulations having low VOC content for the benefit of both the manufacturing site environment and the end user.